1. Field of the Invention
This invention relates to original document scanning devices, and more specifically to original scanning devices having variable scanning speed in copying machines having variable magnification.
2. Description of the Prior Art
Original document scanning devices are employed in various image forming apparatus, such as, for example, electronic copiers. Recently, electronic copiers capable of copying in reduced or enlarged sizes have been placed on the market. The operational principles of such copiers will now be described with reference to FIG. 1. An original document A set on an original table 10 is scanned for exposure by a scanning unit consisting of an exposure lamp 12 and a mirror 14. Light reflected from the original A is directed by mirrors 16 and 18, a lens 20 and a mirror 22 to a uniformly precharged rotating photosensitive drum 24. An electrostatic latent image is thus formed on the photosensitive drum 24. It is then developed, and the developer image thus formed is transferred onto a copying sheet P to complete one copying cycle.
As for the copy size, the dimension of the copy in the scanning direction of the original is determined by the ratio between the scanning speed of the scanning unit and the rotational speed of the photosensitive drum 24 (hereinafter referred to as speed ratio). The dimension of the copy in the direction perpendicular to the scanning direction is determined by the ratio between the distance of the optical path from the original A to the lens 20 and the distance of the optical path from the lens 20 to the photosensitive drum 24 (hereinafter referred to as optical path distance ratio). Accordingly the original A can be copied on a desired scale, enlarged or contracted, by varying the speed ratio and optical path distance ratio.
A problem exists, however, in that many memories are needed to vary the speed ratio. That is, the speed of the pulse motor to drive the scanning unit has to be controlled as shown in FIG. 2 (a), (b) and (c), and copying in reduction and enlargement requires establishment of a speed ratio corresponding to each magnification. Because the constant speed differs for each magnification, the acceleration from stop to a constant speed and deceleration from the constant speed to stop differs for each magnification. To store the control data in the memory for all magnifications requires memorizing a number of items, (acceleration or deceleration steps).times.(numbers of magnifications), resulting in a massive memory. The resulting massive memory requires that other functions of the copying machine be sacrificed.